1. Field of the Invention
The present invention relates to an aluminum paste composition and a solar cell element using the same, and in particular, to an aluminum paste composition for preparing an electrode or a wire on the backside of a silicon semiconductor substrate of a crystal silicon solar cell element, and a solar cell element using the same.
2. Description of the Prior Art
Due to increasingly serious energy shortage and environmental protection problems such as greenhouse effect, many countries have actively developed various alternative energies, among which solar power generation has attracted most attention. FIG. 1 is a schematic cross-sectional structural view of a solar cell element which is fabricated through the following steps. First, a surface of a p-type silicon semiconductor substrate 1 is fabricated into a pyramid-like roughened surface to reduce light reflection (the roughened surface is not shown), and then a reverse conductive n-doped layer 2 is formed onto a light receiving side of the p-type silicon semiconductor substrate 1 by thermally diffusing phosphorous or the like, to form a p-n junction. Then, an anti-reflective layer 3 and an electrode 4 are formed on the n-doped layer 2. A silicon nitride film may be formed on the n-doped layer 2 as the anti-reflective layer 3 by, for example, plasma chemical vapor deposition, and then the front-side electrode 4 is formed by coating a silver conductive adhesive containing silver powder on the anti-reflective layer 3 by screen printing, and then baking, drying and sintering. In the sintering process, the conductive adhesive for forming the front-side electrode 4 may be sintered and penetrate the n-doped layer 2. A backside of the p-type silicon semiconductor substrate 1 is printed with an aluminum conductive adhesive containing an aluminum powder to form an aluminum backside electrode layer 5, and then dried, baked, and sintered at a high temperature. In the sintering process, aluminum atoms diffuse into the p-type silicon semiconductor substrate 1, and an Al—Si alloy layer 6 in alloy state and a p+ layer 7 containing high content of aluminum dopant are formed between the aluminum backside electrode layer 5 and the p-type silicon semiconductor substrate 1. The p+ layer 7 is generally referred to as back surface field (BSF) layer, which can prevent the recombination of electrons and holes, thus facilitating the improvement of the energy transformation efficiency of the solar cell element. Furthermore, in order to serially connect a plurality of solar cell elements to form a module, a silver-aluminum conductive adhesive may be printed onto the aluminum backside electrode layer 5 by screen printing, and then sintered to form a wire 8.
The backside electrode may be formed by printing and drying the silver-aluminum conductive adhesive, and then printing and drying the aluminum conductive adhesive, and baking the two types of conductive adhesives; or by printing and drying the aluminum conductive adhesive, and then printing and drying the silver-aluminum conductive adhesive, and baking the two types of conductive adhesives.
The properties of the aluminum conductive adhesive have great influence on the stability of the solar cell element. If the wettability of the aluminum conductive adhesive to the surface of the substrate is poor, this will result in poor adhesion between the aluminum conductive adhesive and the silicon semiconductor substrate. Moreover, if the difference between the thermal expansion coefficients of the aluminum conductive adhesive and the silicon semiconductor substrate is high, the solar cell element may easily warp. Furthermore, problems such as generation of aluminum beads or bubbles may be caused by different reaction rates of aluminum and aluminum. In the worst case scenario, the above problems may even cause breakage. When the adhesion between the aluminum conductive adhesive and the silver-aluminum conductive adhesive is poor, peeling occurs at the overlap region, which will influence the subsequent process for forming a module by serially connecting a plurality of solar cell elements with the silver-aluminum conductive adhesive if the problem is serious.
Currently known methods for solving the above problems are described in, for example, TW 200713334, TW 200717838, CN 1487531A, CN 1981346A, CN 1877864A, CN 101555388A, and CN 101471389A.
In TW 200713334, an aluminum paste composition is disclosed, which is used to inhibit the generation of bubbles or aluminum particles in an inner electrode layer in sintering, and is characterized by containing an aluminum powder, an organic carrier, and a glass frit. The glass frit accounts for 0.1 to 8 wt % of the aluminum paste composition, and contains 5-75 wt % of an alkali earth metal oxide based on the total weight of the glass frit.
In TW 200717838, a paste composition is disclosed, which is characterized by containing an aluminum powder, an organic carrier, and an adhesion-imparting agent. In this invention, the paste composition containing the adhesion-imparting agent is used to improve the adhesion of an aluminum electrode layer formed on a backside of a silicon semiconductor substrate. The adhesion-imparting agent accounts for 0.05 to 5 wt % of the aluminum paste composition.
In CN 1487531A, an conductive slurry for forming a backside electrode of a silicon solar cell is disclosed, which is used to alleviate warping of silicon chip caused by shrinkage in sintering, and is characterized by containing an aluminum powder, a glass frit, an organic vehicle, and particles slightly soluble or insoluble in the organic vehicle and being at least one of an organic particle or a carbon particle. The glass frit accounts for 1 to 5 wt % of the conductive slurry.
In CN 1981346A, an paste composition is disclosed, which is used to maintain the functions intended to be achieved as a backside electrode of a solar cell element, and enhance the binding of the aluminum electrode layer to the silicon semiconductor substrate, while the content of a glass grit is lowered, or no glass grit is used, and is characterized by containing an aluminum powder, an organic carrier, and a metal alkoxide. The glass grit accounts for 5 wt % or below of the composition.
In CN 1877864A, a slurry composition is disclosed, which has the properties such as good conductivity, high photoelectric conversion efficiency, low warping of silicon chip after sintering, no beads generated, and no bubbles generated, and is characterized by being composed of 70-80 wt % of an aluminum powder, 15-30 wt % of a modified organic binder, and 1-10 wt % of an inorganic binder glass metal powder containing 40-60 wt % of indium, gallium or tantalum.
In CN 101555388A, an inorganic binder for an aluminum slurry is disclosed, which is used to firmly bind an aluminum powder layer onto a silicon chip, and lowers the warping and breakage rate of the cell, and is characterized by being composed of 10-20 wt % of SiO2, 15-30 wt % of B2O3, 5-15 wt % of Al2O3, 15-35 wt % of Bi2O3, 10-18 wt % of Zr2O3, 10-25 wt % of ZnO, and 1-8 wt % of MoO3.
In CN 101471389A, a backside material for a solar cell element is disclosed, which contains a glass mixture, an organic medium, an aluminum-containing material and an additive, and is used to improve the conversion efficiency of the solar cell element and alleviate the problem of warping. The glass mixture contains, for example, Al2O3, Bi2O5, B2O3, SiO2, PbO, Ti2O3, and ZnO, and accounts for 5 wt % or below of the total weight of the backside material.
As described above, in the currently known aluminum paste compositions, different components, such as an adhesion-imparting agent, organic compound particles or carbon particles, a metal alkoxide, special metal ions (e.g. indium, gallium, and tantalum), and special metal oxide (e.g. Zr2O3 and MoO3) are generally added, to inhibit the warping of silicon semiconductor chip, improve the conversion efficiency of the solar cell element, or prevent generation of beads or bubbles in the silicon chip. However, the influence of addition of a dispersing agent on inhibition of the warping of the silicon semiconductor chip, improvement of the conversion efficiency of the solar cell element, or prevention of generation of beads or bubbles in the silicon chip is not disclosed in the prior art. Furthermore, the influence of moisture on the aluminum electrode layer is not disclosed in the prior art, specifically, when the aluminum electrode absorbs the moisture in the air, hydrogen is generated in the packaging process due to the reaction of the moisture and aluminum, which will cause defects after packaging.